The following background discussion includes information that may be useful in understanding the present inventive subject matter. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed inventive subject matter, or that any publication specifically or implicitly referenced is prior art.
Excessive and distracting background noise is a well known issue in many environments, interfering with effective communication, the ability to focus and concentrate, and the enjoyment of recreational activities such as music and film. Historically, efforts at reducing the impact of background noise have focused on occlusion of the ear canal by such means as tightly fitted ear plugs or insulated cups that fit tightly about the outer ear. Commercial headsets, such as those intended for use with mobile telephones or for personal audio players, commonly employ earbuds that cover or are partially inserted into the ear canal. While this positions them advantageously for at least partially blocking background sounds, particularly noisy environments may leave the user with little recourse other than increasing the volume.
More recently some manufacturers, notably Bose™ and Sennheiser™, have introduced specialized headphones or headsets that incorporate active noise cancellation features. In such devices a microphone incorporated into the headset receives background sounds from the environment. This sound is translated into a waveform, which is then processed to generate a sound cancellation waveform that is 180 degrees out of phase with the sound received by the microphone. This sound cancellation waveform is transmitted to speakers incorporated into the headset, where it is expected to reduce the background sounds perceived by the user. The sound cancellation waveform may also be combined with a desired sound signal, such as music, that is transmitted to the speakers simultaneously. Some of these devices, notably the Bose QC1 and QC2, have a feature that allows the user to select between low and high levels of sound cancellation.
The increasing processing power of personal devices such as personal audio players and mobile telephones, has led to the development of software applications for such devices that can generate noise cancelling signals. Such software applications permit the addition of noise cancelling features to a system that incorporates relatively inexpensive general purpose headsets. US patent publication no. 2008/0025523 describes a software application that utilizes a background sound signal obtained from a microphone that is part of a headset to generate a noise cancellation waveform that is 180 degrees out of phase with the background sound within a portable communication device. This noise cancellation waveform is then added to the audio feed supplied to the headset. The application also allows the user to exclude specific sound frequency ranges from noise cancellation.
Such active noise cancellation systems have some degree of effectiveness at reducing unwanted background noise while preserving sounds the user wishes to perceive, however there are drawbacks. Addition of the noise cancelling waveform to the audio feed of a speaker can result in partial cancellation of a desired audio signal, such as music or speech, resulting in undesirable distortion of the perceived sound. Adjustment of the degree of noise cancellation by, for example, the selection of a “low” or “high” setting, only allows a user to reduce this effect in a nonselective manner, and has limited utility at high background noise levels. Exclusion of specific sound frequency ranges from noise cancellation can exacerbate this distortion, in addition to rendering such noise cancellation systems less effective if the nature of the undesired background noise changes.
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Unless the context dictates the contrary, all ranges set forth herein should be interpreted as being inclusive of their endpoints, and open-ended ranges should be interpreted to include commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary.
Thus, there is still a need for a system that can provide effective noise cancellation across a range of environmental conditions while minimizing the loss of perception of desired sounds by the user.